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All marine vessels ranging from a huge cruise liner, big oil tanker, relatively smaller yacht or even a tiny powerboat need some sort of speed control system to control and govern the speed of the marine diesel engine or whatever propulsion plant is being used for the vessel. It would be really impractical and dangerous to have a ship or a boat without speed control mechanism fitted on it, and could lead to accidents such as collision or grounding.

Governor

The speed control is achieved with the help of a governor. Itis necessary to clarify one confusion here:

the main role of the governor is not to increase or decrease the speed which can be done via fuel control system (similar to an accelerator on a car) but

once the speed of the engine has been set, the job of the governor is to maintain that speed despite the variations in load.

Governor

In other words the governor controls the speed variation and keeps the speed within restrained limits despite these variations.

The variations could arise from several factors such as say rough weather. A ship rolling and pitchingin heavy weather may temporarily come in such position that its propeller is literally out of waterand without the governor the speed of the engine could shoot up to such an extent that it could damage the engine itself.

Governor

Governors are also fitted in auxiliary diesel engineson the ship used for power generation, and their function remains the same in this situation as well. The power delivered by the alternator needs to be constant despite load variations and this depends to a great degree on the speed at which the prime mover of the generator diesel engine is rotating since the alternator is getting its movement from that engine only. Hence the role of the governor is equally important in this case as well.

How the governor works

As it has been pointed out, the main function of the governor is to maintain the engine speed at the desired value. It does this by continually positioning the fuel pump racksto control the amount of fuel injected into cylinder per cycle.

Most governors used on diesel engines are self-contained units manufactured by specialist firms. The mechanism can be divided into two parts. There is a speedsensing portionand an actuating portion.

Mechanical governor

The speed sensing part of the mechanical governer consists of a revolving carrieron which are mounted two weights (the flyweights) having cranked levers. They are pivoted so that the levers compress aspringas the weight fly out under centrifugal force.

The force exerted by the spring is controlled by the pressure of an speed adjustment mechanism on its other end. This adjustment is the speed control for the engine. Wherever this is set the weights will assume a corresponding position according to the speed at which they are rotated.

This position is signalled to the actuating portionof the governor by the position of the control spindle.

The control spindleis connected to a piston valve which admits oil as required to the actuating piston and cylinder. The position taken up by this actuating piston is fed back to the pilot valveso that the oil is controlled to give the actuator the desired final position.

The actuator spindle is connected to a lever outputfrom the governor. This lever is connected by the linkage to the fuel pump racksand controls the fuel quantity from each of the fuel pumps.

For any speed setting of the governor there will be a range of output from the fuel pumps according to whether the load torque is high or low. It will vary from full at one end of the travel of actuatorpistontono fuel at the other. These two positions correspond to two slightly different values of speed. The difference between these values is known as the governor droop. It is usually expressed as percentage of the set speed.

The linkage to the fuel pumps may take the form of a push-pull mechanism. Each individual fuel pump rack is connected to the mechanism by an adjustable link so they can be balanced to ensure that each cylinder takes its share of the load. The connection between each individual pump and common linkage incorporates a springarranged as a “fail safe” device so that should one pump stick in a fuel supply position the others can be safely returned to the no fuel position.

It is also the usual practice to provide a manualoverride by which the linkage can be returned to the no fuel positionin emergency or should the governor fail in any way. Springs in the mechanism between the governor and the main fuel rack rod enable this operation to be carried out.

Exercise 1 – Answer the following questions:

What is the function of the governor?

What are the two main parts of the governor?

Describe the operation of the sensing portion.

How is the speed of the engine maintained at the desired level?

What is the actuating portion linked to? What does the linkage consist of?

What is the governor droop?

What is the usual droop value?

How does each cylinder take a part of the propulsion load?

When is the manual override used?

Why are overspeed trips fitted to the engines ?

Exercise 2 – Say which of the statements given below are TRUE and which are FALSE. If FALSE, state why:

Governors for diesel engines are usually made up of two systems: a mechanical arrangement and hydraulic unit.

The mechanical portion of the governor acts directly on the fuel control to change the engine power output.

The hydraulic unit detects any change in the speed of the engine and transfers the indication to the actuator.

The rotation of the flyweights produces a centrifugal force which is opposed and balanced by a spring.

If the engine speed slows down, the centrifugal force on the flyweights increases and the crank levers compress the spring.

Exercise 2 – Say which of the statements given below are TRUE and which are FALSE. If FALSE, state why:

The set speed may be changed by the speed control which alters the spring compression.

A piston valve connected to the control spindle admits or cuts immediately the fuel supply from each of the fuel pumps.

When the piston valve rises, oil is drained from the actuating cylinder and the power piston moves up increasing the fuel supply to the engine which speeds up.

The actuating piston is spring loaded and when the engine speed increases, the oil pressure is released allowing the spring to force the piston down thus decreasing the fuel supply.

Exercise 3 – Complete the following sentences with the appropriate terms:

The device consisting of a pair of identical flyweights, mounted on opposite sides of revolving carrier is the ____________ .

The adjustment of the spring force which resists the centrifugal forces of the flyweights is the ____________ of the engine.

The piston acted upon by oil under pressure which actuates the fuel control mechanism of the engine is the ____________ .

The small cylindrical valve that slides up and down in a bushing containing ports wich control the oil flow to and from the actuating cylinder is called the ___________ .

Exercise 3 – Complete the following sentences with the appropriate terms:

The rod transmitting the position assumed by the flyweights to the pilot valve is known as the _________.

The straight bar having teeth suitable for engaging with a pinion used for setting the fuel supply is the _________.

The change in the governor rotating speed which causes the governor’s fuel control mechanism to move from its full-open position to its full-closed position is called ____________.

A manually operated device that enables the fuel supply mechanism to be returned to no fuel position in case of emergency or governor’s failure is known as the ____________.

Exercise 4 –Name the basic elements of the mechanical governor.

a – set screwb –

Exercise 5

Now do the following exercises:

Say which components form the speed sensing arrangement and which the actuating system.

State the connections between: (a) the speed sensing and actuating system, (b) the actuating and fuel control system.